Fix issue Zulko#2247

Fixes the issue when concatenating videoclips as mask is being considered as [1, 1] so the mask shape is different from the other videos and fails.

I think [1, 1] could be a default mask to avoid divisions by zero or something similar, but here it does not make sense.

Issue here: Zulko#2247

moviepy/video/compositing/CompositeVideoClip.py

@@ -289,7 +289,7 @@ def frame_function(t):
             return clips[i].get_frame(t - timings[i])
 
         def get_mask(clip):
-            mask = clip.mask or ColorClip([1, 1], color=1, is_mask=True)
+            mask = clip.mask or ColorClip(clip.size, color=1, is_mask=True)
             if mask.duration is None:
                 mask.duration = clip.duration
             return mask